Protection of carbon articles

ABSTRACT

Carbon articles, particularly carbon electrodes, are protected against corrosive attack in use by fusing material to the article, or applying material to the carbon article which under the conditions of its use is capable of fusing to the carbon article, to form a coherent coating of protective material over only that part of the carbon article liable to corrosive attack when the article is in use.

This is a continuation of Ser. No. 518,962, filed Oct. 29, 1974 nowabandoned as a division of Ser. No. 343,217, filed Mar. 21, 1973, nowabandoned.

This invention relates to the protection of carbon articles, e.g.graphite crucibles, furnace linings etc. against attack, e.g. againstoxidation when subjected to elevated temperatures. In particular thepresent invention relates to the reduction of oxidation losses ofgraphite electrodes such as furnace electrodes.

It has been shown that when a graphite electrode is used in the electricmelting of steel in arc furnaces substantial electrode losses areencountered due to oxidation of the sides of the electrodes during use.As electrode costs are a very substantial part of the electric arcsteelmaking process, such losses are very costly. In severe cases, up to70% by weight of the electrode may be oxidised from the side of theelectrode and not in the arc itself. In view of the above many attemptshave been made to protect arc furnace electrodes by protective coating,but until now these have met with very little success. Among attemptswhich have been made are those of British Patent Specifications Nos.1,026,055, 1,218,662 and German Pat. No. 1,009,093. These specificationsdescribe coatings of one or more layers including aluminium withboron/silicon/titanium alloys or compounds, iron withchromium/aluminium/silicon alloys or compounds, and titaniumsilicide/silver.

In all these processes, the electrodes are coated prior to being put toservice in the furnace or the like. This leads to difficulties, becausethe electrode clamps, which supply current to the electrodes, cannotmake direct electrical contact with the carbonaceous electrode material.Consequently the current has to pass through the applied coating, andsince such coatings often have specific resistances higher than those ofthe clamp and electrode materials, the coating in the region of theclamp becomes locally overheated. This can cause burning of the clampsthemselves, and disruption of the coating so that when this part of theelectrode is lowered, with respect to the clamp, into the furnace, thedegree of protection is considerably reduced. Local overheating can alsobe caused if the coatings are uneven or have been damaged during themounting of the electrode in the clamp, or, as may be the case withcertain metallic coatings, by reaction or alloying with the clamp.Further, since the coatings suggested so far have normally beenrefractory and brittle, they do not readily accommodate changes involume due to temperature fluctuations and tend to crack. Oxidation ofthe electrode then proceeds through the crack under the coating.Coatings which have failed, either in this way or by other degradationsuch as melting, evaporation or oxidation, are difficult to repair sincethe application processes do not lend themselves to repeated coating insitu.

According to the present invention there is provided a method ofprotecting a carbon article against corrosive attack in use which methodcomprises fusing material to the carbon article, or applying material tothe carbon article which under the conditions of its use is capable offusing to the carbon article, to form a coherent coating of protectivematerial over only that part of the carbon article liable to corrosiveattack when the article is in use.

The present method is of particular utility in the protection of carbon,e.g. graphite electrodes against oxidation. In this case material isfused to or applied to the electrode so as to form a coherent coating ofprotective material over only that part of the electrode liable tooxidation when the electrode is in use. Generally the material is fusedto or applied to the electrode such as to form a coherent coating oversubstantially all of that part of the electrode not held by clamps whenthe electrode is in use, i.e. that part within the furnace itself andimmediately above, but below the position of the clamps. Thus all ormost of the electrode surface below the clamps will at any time beprotected, but the clamps themselves will always maintain electricalcontact with the electrode surface directly avoiding local overheating.Furthermore, the nature of the protective material and its method ofapplication enable further treatments of the electrode to be made asnecessary, for example when the electrode is lowered as consumptiontakes place at the tip then fresh electrode must be protected, or tostrengthen the initial coating to compensate for gradual degradation dueto evaporation and/or other losses.

The present invention also provides a process for melting metal using acarbon electrode electric arc furnace which process comprises:

(a) fusing material to electrodes of the furnace, or applying toelectrodes of the furnace material which under the conditions of step(b) is capable of fusing to the electrodes, to form a coherent coatingof protective material only over that part of the electrode which isliable to oxidation in step (b) and

(b) melting metal in the furnace by applying electric current to theelectrodes.

The invention also provides a carbon electrode carrying a coherentcoating of protective material over only that part of the electrodeliable to attack when the electrode is in use.

The protective material is generally applied to the carbon articles,e.g. electrodes, in the form of pre-formed sheet material, e.g. in theform of tubes, tiles or plates comprising two basic components, a matrixpreferably having a melting point below 1000° C. and a refractoryfiller. These sheets are usually 1 to 10 mm thick. Optionally the sheetsmay also contain fibrous material, which has been found to impartstrength and flexibility to the finished article, and in addition may beadvantageous for certain manufacturing processes.

The components of the sheet material may be mixed, melted or fusedtogether, optionally with a binder, and formed into the desired shape bycasting, pressing, rolling or extruding, for example. Any of theseforming processes may be carried out with the application of heat (hotforming) if desired. Alternatively, the components may be mixed with aliquid or plastic medium which may, optionally, act as a binder, to forma paste or liquid which is then formed into shape by a process such asone of those mentioned above, this forming process may be followed by adrying, or curing procedure, if necessary.

A preferred method of forming the sheets is to mix the basic componentsinto a suspension or slurry with a liquid carrier, usually water. Thisslurry may also advantageously contain fibrous material. The slurry isthen dewatered by filtering through a gauze or similar pervious surface,this filtration being preferably aided by a pressure difference whichmay be established by increasing the pressure above the slurry and/or byreducing the pressure (i.e. evacuation) on the efflux side of the gauzeor filter. After the filtration is complete the solids remain as a moistthin cake on the filter. This cake may then be removed for drying and/orfurther treatment.

This drying process is conveniently carried out by placing the "green"sheets, tiles or tubes on drying trays or racks, which may be flat orshaped. In the case of shaped racks, these may impart to the dried sheetthe approximate curved form of the electrodes to which they are to beapplied. Alternatively, such shaped sheets or tiles may be produced byforming the green sheets or tiles to a self-supporting shape, e.g. acylinder or semi-cylinder either by using a dewatering filter of theappropriate curvature or by shaping the green sheets or tiles afterdewatering. Sheets, tiles and tubes are hereinafter collectivelyreferred to as tiles.

Tiles made by any of the above processes may optionally incorporate oneor more layers of more flexible material such as for example plastics,cardboard, heavy paper, ceramic fibre "paper" or other fibrous sheetmaterial, or thin sheet metal, in order to improve the strength andflexibility characteristics of the finished sheets or tiles. Suchadditional layers do not necessarily contribute to the protectiveproperties of the complete tiles but, in those cases where theadditional layer is present on the outer surface of the tile, and thetile becomes sticky during application, the layer prevents the tile fromsticking to the application equipment. The additional layers may beadvantageously incorporated during the manufacturing process but mayalternatively be applied to the tile, for example by adhesive, afterformation of the tile is complete.

After application to the electrode and/or when the electrode is used inthe furnace, the matrix fuses, to form a continuous coating sticking tothe electrode surface. This molten matrix constitutes a layer of highimpermeability, which property is further enhanced by the presence ofthe filler. Such a coating substantially reduces attack of the electrodeby the furnace atmosphere.

A further very important property of the filler is to increase theviscosity of the matrix so that it does not drip off from the electrode,and the filler level is so adjusted that the coating is fluid/plasticover a wide temperature range thus easily accommodating temperaturefluctuation without cracking.

It has been found that the matrix to filler weight ratio in the sheetsor tiles may vary between 90:10 and 15:85 depending upon the temperatureto which the applied sheets are to be subjected. For the purposes ofclarification, preferred matrix/filler weight ratios in the presentinvention are:

For a service temperature of 700° C. a matrix/filler weight ratiobetween 65:35 and 85:15 is preferred.

For a service temperature of 1600° C. a matrix/filler weight ratiobetween 25:75 and 50:50 is preferred.

The matrix may contain "combined water" which is driven off immediatelyafter application of the sheets or tiles to the electrode. An example ofsuch a matrix is boric acid which, in use, is converted to boric oxide.

Thus a sheet or tile having an original matrix to filler weight ratio of80 boric acid:20 filler is converted into a coating having a weightratio of 70:30 boric oxide/filler.

In many cases it may be advantageous, particularly if the sheets ortiles are prepared from a slurry, to include fibrous material of one ormore types to a total amount not normally exceeding 20% by weight.

Because the coating firmly adheres by virtue of the wetting of theelectrode surface by the matrix, and because there is no tendency tocrack, further sheets or coating may be subsequently applied withoutrisk of the initial layer flaking off.

This adhesion may be enhanced by the application of a coating of thematrix material or of matrix enriched material or of a similar materialwhich acts as an adhesive. Such adhesive coatings may be convenientlyapplied to the appropriate surface of the sheet during or following theforming process and/or directly to the surface of the electrode.

The sheets may be applied to the hot electrode either above or below theroof of the furnace or adjacent to the furnace and held in position byexternal means until the protective material has softened sufficiently,by melting of the matrix, to allow the sheet to take up the exactcontour of and to adhere to the surface of the electrode.

Should it not be practical in certain furnace constructions to apply thesheets whilst the electrode is in position in its clamp, then theelectrode may be removed from its clamp for application of the sheetsaway from the furnace; if the electrode is removed from its clamp forthis purpose the sheets are applied to only that part of the electrodebelow the clamping level.

Furthermore, if that part of the electrode to which the protectivematerial is to be applied is at a temperature below that of the meltingpoint of the matrix material or other adhesive materials, the sheets maybe conveniently held in position by, for example, bands, clips, wire,nails or the like until the electrode attains the temperature, necessaryto render the sheet self-adhesive, e.g. by being brought into use in thefurnace.

Alternatively, the electrode may be heated by external means before orafter application of the sheets thereto, e.g. by flame or infraredheating, or by electrical heating such as induction or microwaveheating, so that at least the surface of the electrode attains thetemperature necessary to render the sheet self-adhesive.

Typical materials available for the manufacture of the sheets are asfollows:

Suitable matrices may be chosen from graphite-wetting materials forexample

(1) boron-containing compounds such as boric oxide, boric acid,metaboric acid, salts of these acids e.g. sodium borate;

(2) vanadium pentoxide;

(3) combinations of glaze-forming materials e.g. phosphates, fluoridesor silicates such as alkali metal phosphates, aluminium orthophosphate,alkali metal silicates, glass, calcium fluoride, sodium aluminiumfluoride or sodium borofluoride.

It is possible to produce satisfactory products by combining into acomposition one or more materials from group 3 with one or more of thematerials of group 1 and/or 2 or it may be desirable to use one or moreof any of the materials included in groups 1-3 in conjunction with asurface tension modifying agent such as chrome ore.

Suitable fillers may be, for example refractory oxides, carbides,nitrides or borides such as chromic oxide, magnesium oxide, zirconiumoxide, titanium oxide, silicon carbide, wolfram carbide, boron nitride,silicon nitride, titanium boride, zirconium boride and zirconiumcarbide. The refractory filler material generally comprises 80% ofparticles having a particle size of less than 0.5 mm, preferably 80%particles less than 0.2 mm in size and more preferably 80% particlesless than 0.06 mm in size.

Suitable fibrous materials include asbestos, aluminium silicate fibre,glass fibre, aluminium chromium silicate fibre, calcium silicate fibre,mineral wool, slag wool, paper including waste paper, paper pulp andnewsprint, and textile fibres such as rayon, cotton and the like.

Especially effective protective coating compositions are thosecomprising matrices based on boron compounds with refractory carbidefillers. In particular coatings comprising a matrix of boric oxide orboric acid with a silicon carbide filler have proved especiallyeffective, initial trials having shown reductions in electrodeconsumption of up to 52%.

The following Examples further illustrate the present invention.

EXAMPLE 1

To a hot electrode (1200° C.) measuring 230 mm diameter for use in a 10ton capacity furnace there were applied sheets measuring 25 cm squarecomprising 65% silicon carbide, 29% boric acid (H₃ BO₃) (80:20 siliconcarbide to boric oxide weight ratio after application), 4% amositeasbestos and 2% chopped (1 cm) rayon fibres. Each sheet was coated,after forming and drying, with boric acid at a rate of 4 g per sq.dm. Onfusing onto the surface of the electrode a coating approximately 2 mmthick was produced.

After 2 charges (approximately nine hours) in comparison to the uncoatedpart of the electrode it was found that electrode consumption had beenreduced by 52%.

EXAMPLE 2

To a hot electrode (1050° C.) measuring 560 mm diameter for use in a 70ton capacity furnace there were applied sheets measuring 25 cm squarecomprising 54% silicon carbide, 40% boric acid (H₃ BO₃) (70:30 siliconcarbide to boric oxide weight ratio after application), 4% amositeasbestos and 2% chopped (1 cm) rayon fibres. Each sheet was coated,after forming and drying, with boric acid at a rate of 4 g per sq.dm. Onfusing onto the surface of the electrode a coating approximately 2 mmthick was produced.

After 4 charges (approximately 17 hours) in comparison to the uncoatedpart of the electrode it was found that electrode consumption had beenreduced by 48%.

We claim:
 1. An article consisting of a self-supporting pre-formedelectrode protection sheet, the composition of the sheet consistingessentially of a matrix material of boric oxide which is agraphite-wetting material and silicon carbide as a refractory filler,the boric oxide and silicon carbide being bonded to coherent sheet form.2. An electrode protection sheet according to claim 1 wherein therefractory filler consists to an extent of at least 80% of its weight ofparticles of size less than 0.2 mm.
 3. An electrode protection sheetaccording to claim 1 wherein the refractory filler constitutes 10 to 85%by weight of the sheet.
 4. An electrode protection sheet according toclaim 1 and including up to 20% by weight fibrous material.
 5. Anarticle consisting of a self-supporting pre-formed electrode protectionsheet, the composition of the sheet consisting essentially of a matrixmaterial of boric acid which is a graphite-wetting material and siliconcarbide as a refractory filler, the boric acid and silicon carbide beingbonded to coherent sheet form.
 6. An electrode protection sheetaccording to claim 5 wherein the refractory filler consists to an extentof at least 80% of its weight of particles of size less than 0.2 mm. 7.An electrode protection sheet according to claim 5 wherein therefractory filler constitutes 10 to 85% by weight of the sheet.
 8. Anelectrode protection sheet according to claim 5 and including up to 20%by weight fibrous material.
 9. An article consisting of aself-supporting pre-formed electrode protection sheet, the compositionof the sheet consisting essentially of a matrix material which is agraphite-wetting material and a refractory filler, the matrix materialbeing selected from the group consisting of boric oxide, boric acid,metaboric acid, salts of boric acid and metaboric acid, vanadiumpentoxide and mixtures thereof.
 10. An article consisting of aself-supporting pre-formed electrode protection sheet, the compositionof the sheet consisting essentially of a matrix material which is agraphite-wetting material and a refractory filler, the matrix materialbeing also a glaze-forming material selected from the group consistingof phosphates, fluorides, silicates and mixtures thereof.
 11. An articleconsisting of a self-supporting pre-formed electrode protection sheet,the composition of the sheet consisting essentially of a matrix materialwhich is a graphite-wetting material and a refractory filler, the matrixmaterial being a mixture of graphite-wetting materials selected from thegroup consisting of boric oxide, boric acid, metaboric acid, salts ofboric acid and metaboric acid, vanadium pentoxide and mixtures thereof,and graphite-wetting materials which are also glaze-forming materialsselected from phosphates, fluorides, silicates and mixtures thereof. 12.An electrode protection sheet having a thickness in the range 1 to 10 mmsaid sheet consisting essentially of a bonded sheet formed of acomposition consisting essentially of a matrix material and a refractoryfiller, wherein the matrix material is a mixture of graphite-wettingmaterials selected from the group consisting of boric acid, metaboricacid, salts of boric acid and metaboric acid, vanadium pentoxide andmixtures thereof, and graphite-wetting materials which are alsoglaze-forming materials selected from phosphates, fluorides, silicatesand mixtures thereof, and the material of the sheet being heat-fused tothe surface of a graphite electrode.
 13. An electrode protection sheetformed of a bonded composition consisting essentially of an inorganicgraphite-wetting matrix material and a particulate refractory filler,said sheet being heat-fused to an exterior surface of a graphiteelectrode.